The present invention relates to a color display tube having an in-line electron gun equipped with a field controller, and in particular to a color display tube optimum to a color monitor display device of high resolution in which the electron beam is deflected with a horizontal scanning frequency higher than the standard horizontal scanning frequency.
In a color cathode-ray tube having an in-line electron gun, three electron beams are arranged on a line in a coplane. Therefore, two exterior beams among the in-line beams are eccentric with respect to the electro-magnetic deflection center. The electron beam passing through the convergence electrode is deflected by the leakage magnetic field originating from the deflection yoke. At this time, the magnetic flux of the leakage magnetic field is not uniform over the section of the electron gun. Accordingly, the amount of deflection (deflection sensitivity) of the center beam is different from that of the exterior beams. As a result, the shape of the raster formed by the scanning of the center beam (green) is different from that formed by the scanning of each of the exterior beams (red and blue). The so-called coma aberration is generated, resulting in poor color reproduction at comparatively exterior parts on the screen.
In order to compensate the coma aberration, a field controller for controlling the magnetic flux distribution of the leakage magnetic flux at the rear end side of the deflection yoke is disposed in a region through which the electron beam passes. This field controller is made of a magnetic material having high permeability.
A color display tube having such a field controller is disclosed in Japanese Examined Patent Publication No. 26208/76 assigned to the Tokyo Shibaura Electric Industrial Company and filed May 18, 1971, for example.
In the standard color television system, 15.75 kHz is generally used as the horizontal scanning frequency f.sub.H. In recent years, higher resolution is demanded for the picture on the display monitor device of the computer terminal. Therefore, a higher frequency as compared with the standard system tends to be chosen as the horizontal scanning frequency. As a result, coma distortion is caused. In particular, especially large distortion is caused on the left side of the screen, resulting in a problem.
The present inventors took note of this phenomenon and conducted experiments. As a result, the present inventors found that a higher deflection field frequency deteriorates the magnetic characteristics of the field controller and hence the desired compensation function for the leakage magnetic field is lost, resulting in the above described problem. That is to say, the increase in the deflection magnetic field frequency (horizontal scanning frequency) causes an increase in eddy-current loss of the field controller. Thus the permeability of the field controller is lowered and hence the effect of the magnetic shield or the magnetic enhancement is deteriorated. When the horizontal scanning frequency is raised in the case of the field controller functioning the magnetic shield, the deflection amount of the center beam becomes small and the deflection amount of the exterior beam becomes large. In the case of the field controller functioning the magnetic enhancement the contrary becomes true. Further, an increase in the horizontal scanning frequency shortens the horizontal . retrace line time. Due to the magnetic aftereffect, the amount of misconvergence between the center beam and the exterior beams becomes large especially on the left side of the screen.